Method of fabricating an integrated circuit with a pattern density-outlier-treatment for optimized pattern density uniformity

ABSTRACT

The present disclosure provides one embodiment of an IC method. First pattern densities (PDs) of a plurality of templates of an IC design layout are received. Then a high PD outlier template and a low PD outlier template from the plurality of templates are identified. The high PD outlier template is split into multiple subsets of template and each subset of template carries a portion of PD of the high PD outlier template. A PD uniformity (PDU) optimization is performed to the low PD outlier template and multiple individual exposure processes are applied by using respective subset of templates.

PRIORITY DATA

This application is a continuation of U.S. patent application Ser. No.14/483,893, filed on Sep. 11, 2014, which will issue as U.S. Pat. No.9,552,964, which claims priority to U.S. Provisional Patent ApplicationNo. 62/014,997, filed on Jun. 20, 2014, the entire disclosures of whichare herein incorporated by reference.

BACKGROUND

In integrated circuit (IC) manufacture, it is common to utilize opticalproximity correction (OPC) to improve an imaging resolution of an ICpattern during a lithography patterning process. However, along with theprogress of semiconductor technology, the feature sizes are continuallygetting smaller. The existing OPC methods to add various dummy featureshave a limited degree of freedom and effectiveness to tune the patterndensity and poor uniformity of the pattern density. This presents issuessuch as space charge effect and micro-loading effect when anelectron-beam lithography technology is used to form the IC pattern.Furthermore, during the process to insert dummy features, varioussimulations and calculations associated with the dummy features takemore time, causing an increase in cost. Therefore, what is needed is amethod for IC design and mask making to effectively and efficientlyadjusting an IC pattern to address the above issues.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a flowchart of an embodiment of an integrated circuit (IC)method in accordance with some embodiments.

FIG. 2 is a schematic view of a semiconductor substrate constructed inaccordance with some embodiments.

FIG. 3 is a schematic view of the semiconductor substrate of FIG. 2, inportion, constructed in accordance with some embodiments.

FIG. 4 illustrates an integrated circuit (IC) design layout constructedin accordance with some embodiments.

FIG. 5 illustrates a pattern density constructed in accordance with someembodiments.

FIGS. 6-11 illustrate patterns of a template of an IC design layoutconstructed in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

FIG. 1 is a flowchart of a method 100 for fabricating an integratedcircuit (IC) according to various aspects of the present disclosure inone or more embodiments. The method 100 is described with reference to adevice shown in FIGS. 2 through 11.

First, an IC design layout is provided by a designer. In one example,the designer is a design house. In another example, the designer is adesign team separated from a semiconductor manufacture assigned formaking IC products according to the IC design layout. In variousembodiments, the semiconductor manufacturer is capable for makingphotomasks, semiconductor wafers, or both. The IC design layout includesvarious geometrical patterns designed for an IC product and based on thespecification of the IC product.

The IC design layout is presented in one or more data files having theinformation of geometrical patterns. In one example, the IC designlayout is expressed in a GDS or GDS-II format, as well known in the art.The designer, based on the specification of the IC product to bemanufactured, implements a proper design procedure to generate the ICdesign layout. The design procedure may include logic design, physicaldesign, and place and route. As an example, a portion of the IC designlayout includes various IC features (also referred to as main features),such as active regions, gate electrodes, source and drains, metal lines,contacts/vias, and openings for bonding pads, to be formed on asemiconductor substrate (such as a silicon wafer) or on various materiallayers disposed over the semiconductor substrate. The IC design layoutmay include additional features, such as those features for imagingeffect, processing enhancement, and/or mask identification information.

FIG. 2 illustrates a schematic view of a semiconductor substrate 120constructed in accordance with some embodiments. In the presentembodiment, the semiconductor substrate 120 is a semiconductor wafer,such as silicon wafer. In other embodiments, the semiconductor substrate120 may alternatively or additionally include other semiconductormaterial, such as germanium (Ge), silicon germanium (SiGe), siliconcarbide (SiC), indium arsenide (InAs), or indium phosphide (InP), or asuitable alloy semiconductor, such as silicon germanium carbide, galliumarsenic phosphide, or gallium indium phosphide. The semiconductorsubstrate 120 may include various doped regions, dielectric features,and multilevel interconnects. In one embodiment, the semiconductorsubstrate 120 includes various doped features for variousmicroelectronic components, such as a complementarymetal-oxide-semiconductor field-effect transistor (CMOSFET), imagingsensor, memory cell, and/or capacitive element. In another embodiment,the semiconductor substrate 120 includes conductive material featuresand dielectric material features configured for coupling and isolatingvarious microelectronic components, respectively. In another embodiment,the semiconductor substrate 120 includes one or more material layers(such as a dielectric material layer) formed thereon.

The semiconductor substrate 120 further includes various circuitregions, represented by a single region 122 as shown in FIG. 2, althoughmore regions can exist on the substrate. One or more ICs are to beformed in each circuit region 122, separated from each other by scribelines. In one embodiment, an IC design layout for a single IC chip is tobe formed in each of the circuit regions 122. For the sake of furtherexample, and not intended to be limiting, the following discussion willrefer to a one-chip region, as IC chip 122.

Referring to FIG. 3, each IC chip 122 is divided into a plurality ofareas (also referred to as templates) 124 for subsequent operations. Inthe present embodiment, the templates 124 each have an equal area. Thenumber of templates 124 in the full chip is “N”. The templates 124 arereferred to as 1, 2, 3, . . . , i, . . . and N, respectively. In thepresent example, each template 124 includes a rectangle or a squareregion in the semiconductor substrate 120. The number N may bedetermined by template size, which is closely related to e-beam modelambit (electron diffusion or convolution kernel range in μm). The numberN may also be determined according to one or more factors, such ascalculation efficiency. When the number N is large, the operations inthe following process may take a longer time to complete. When thenumber N is small, the operations in the following process may take lesstime to complete, but may result in reduced optimization effectiveness.Accordingly, the number N is properly chosen according to one or morefactors, such as engineer experience and/or previous processed data(such as historic data collected from execution of the method 100.

Referring to FIG. 4, the IC design layout to be formed on the chip isdefined in various templates 124. Each template 124 includes a portionof the IC design layout. The IC design layout defined in differenttemplates may be different from each other, depending on individualintegrated circuit and the corresponding IC design layout.

The IC design layout includes main features 132 designed and configuredto form a portion of the integrated circuit. A main feature is ageometrical pattern that defines an IC feature, such as contact/viahole, to be formed on the semiconductor substrate 120. A space isolationdimension (simply referred to as isolation distance) “d” is a parameterto define a forbidden area 136 surrounding a main feature 132, in whichfeatures (excluding dummy features) should not be inserted. The ICpattern in the template 124 includes a plurality of main features 132and accordingly a plurality of forbidden areas 136 surrounding therespective main features 132. By excluding the main features 132 and theforbidden areas 136, the remaining regions in the semiconductorsubstrate are defined as space block(s) 138 for dummy insertion. The ICpattern includes the main features 132, the forbidden areas 136 andspace block(s) 138.

Referring again to FIG. 1, the method 100 begins at step 102 byproviding a main pattern density (PD) 210 of each template 124. Themethod 100 then proceeds to step 104 by choosing a PD target 310 with aPD target range R_(t) for templates 124 and identifying PD outliertemplates 124 _(outlier) according to the PD target 310 and the PDtarget range R_(t).

In one embodiment, a PD analysis is performed first. For example, a PDhistogram is generated (as shown in FIG. 5) and then a range R of the PDis calculated, which is from a lowest PD 210 (referred to as PD 210L) toa highest PD 210 (referred to as PD 210H). In the present embodiment,the PD target 310 is chosen to be smaller than a maximum PD defined bye-beam blur budget, referred to as PD_(blur). In one embodiment, the PDtarget 310 is determined to be smaller than the PD_(blur) and equal tothe PD 210, which locates in half of the range R. The PD target rangeR_(t) is then chosen to be substantial smaller than the range R. In oneembodiment, the PD target range R_(t) is chosen as +/−10% of the PDtarget 310.

After the PD target 310 and the PD target range R_(t) are chosen, thetemplate 124 having a PD that falls out of the PD target range R_(t) isdefined as the PD outlier template 124 _(outlier). A template 124 isreferred to as a low PD outlier template 124 _(outlier) when its PD islower than the PD target 310, now labeled with a reference number 320L.Similarly a template is referred to as a high PD outlier template 124_(outlier) when its PD is higher than the PD target 310, now labeledwith a reference number 320H.

Referring again to FIG. 1, the method 100 proceeds to step 106 byperforming a PD-outlier-treatment to the respective PD outlier template124 _(outlier). The method 100 has three paths starting from step 106,identified by the suffix “A”, “B” and “C” respectively. The path A andpath B provide two different PD-outlier-treatments for the high PDoutlier template 320H and the path C provides a PD-outlier-treatment Cfor the low PD outlier template 320L. Every path is separately discussedbelow.

Referring to FIGS. 1 and 6, in path A, the method 100 proceeds to step106A by performing a PD-outlier-treatment A to the high PD outliertemplate 320H. In one embodiment, the high PD outlier template 320Hincludes a first region 321 and a second region 322. The first region321 and the second region 322 have a quite different pattern criticaldimension (CD). For example, the first region 321 has a quite largepattern CD, such as 600 nm, while the second region 322 has a quitesmall pattern CD, such as 30 nm. In the PD-outlier-treatment A, thefirst region 321 is split into a first subset of template 320HA, and thesecond region 322 is split into a second subset of template 320HB.

Referring to FIGS. 1 and 7, continuing in path A, the method 100proceeds to step 108A by performing pattern density uniformity (PDU)optimization to the second subset of template 320HB and leaving thefirst subset of template 320HA with no further treatment at this step.The PDU optimization is described in the above-listed application thathas been incorporated by reference. For example, in the PDUoptimization, sub-resolution dummy features 410 are generated andinserted in the second subset of template 320HB to increase its PD, nowlabeled with a reference number 320HC. A size (and pitch), shape andtype of sub-resolution dummy features 410 are chosen to make the PD ofthe subset of template 320HC satisfy the PD target 310 with the PDtarget range R_(t).

Referring again to FIG. 1, the method 100 proceeds to step 110A byperforming a first exposure process to the semiconductor substrate 120using the first subset of template 320HA and a second exposure processto the semiconductor substrate 120 using the second subset of template320HC, with different exposure dose than an exposure dose used in thefirst exposure process.

Referring to FIGS. 1 and 8, in path B, the method 100 proceeds to step106B by performing the PD-outlier-treatment B to the high PD outliertemplate 320H. In one embodiment, the high PD outlier template 320Hincludes a third region 323 and a fourth region 324. Both of the thirdregion 323 and the fourth region 324 has quite small pattern CDs. Forexample, a pattern CD of the third region 323 is about 50 nm and apattern CD of the fourth region 324 is about 30 nm. Also, a high PD ofthe high PD outlier template 320H is mainly contributed by a PD of thethird region 323. In the PD-outlier-treatment B, the high PD outliertemplate 320H is split into a multiple of subsets of template and eachof subset of template carries a portion of the PD of the high PD outliertemplate 320H. Each PD of subsets of template satisfies the PD target310, or is within the PD target range R_(t).

In one embodiment, the high PD outlier template 320H is split into twosubsets of template: a third subset of template 320HD and a fourthsubset of template 320HE. The third subset of template 320HD carriersX₁% of the PD of the third region 323, referred to as 323A, and 100% ofthe PD of the fourth region 324. The fourth subset of template 320HEcarries X₂% of the PD of the third region 323, referred to as 323B. Asum of the X₁% and X₂% of the PD of the third region 323 is equal to100% of the PD of the third region 323. Each of PDs of the third subsetof template 320HD and the fourth subset of template 320HE satisfies thePD target 310, or is within the PD target range R_(t).

In another embodiment, alternatively, the high PD outlier template 320His split into three subsets of template: a fifth, a sixth and a seventhsubset of template, 320HF, 320HG and 320HH, respectively, as shown inFIG. 9. The fifth subset of template 320HF carries X₃% of a PD of thethird region 323, referred to as 323C, and the sixth subset of template320HG carries X₄% of the PD of the third region 323, referred to as323D. A sum of the X₃% and X₄% of the PD of the third region 323 isequal to 100% of the PD of the third region 323. In the meantime, eachof the X₃% and the X₄% of the PD of the third region 323 satisfies thePD target 310, or is within the PD target range R_(t). The seventhsubset of template 320HH carries 100% of the fourth region 324.

Referring to FIGS. 1 and 10, the method 100 proceeds to step 108B byperforming a PDU optimization to the seventh subset of template 320HHand leaving the fifth and sixth of subsets of template, 320HF and 320G,(as well as the third and the fourth of subsets of template, 320HD and320HE) with no further treatment at this step. The PDU optimization isdescribed in the above-listed application that has been incorporated byreference. For example, in the PDU optimization, sub-resolution dummyfeatures 420 are generated and inserted in the seventh subset template320HH to increase its PD, now labeled with a reference number 320HI. Asize (and pitch), shape and type of sub-resolution dummy features 420are chosen to make the PD of the seventh subset template 320HI satisfythe PD target 310 with the PD target range R_(t)

Referring again to FIG. 1, the method 100 proceeds to step 110B byperforming individual exposure process to the semiconductor substrate120 by using each of subsets of template. In one embodiment, a firstexposure process is performed to the semiconductor substrate 120 usingthe third subset of template 320HD and a second exposure process isperformed to the semiconductor substrate 120 using the fourth subset oftemplate 320HE, with different exposure doses than to the first exposureprocess. In another embodiment, a third exposure process is performed tothe semiconductor substrate 120 by using the fifth subset of templates320HF, a fourth exposure process is then performed to the semiconductorsubstrate 120 by using the sixth subset of templates 320HG and a fifthexposure process is performed to the semiconductor substrate 120 byusing the seventh subset of templates 320HI. Each of exposure dose ofthe third, fourth and fifth exposure process is different to each other.

Referring to FIGS. 1 and 11, in path C, the method 100 proceeds to step106C by performing the PD-outlier-treatment C to the low PD outliertemplate 320L. The low PD outlier template 320L may includes a fifthregion 325 having main patterns and a background region 326 without mainpatterns. A PDU optimization is performed to the low PD outlier template320L. The PDU optimization is described in the above-listed applicationthat has been incorporated by reference. For example, in the PDUoptimization, sub-resolution dummy features 430 are generated andinserted in the background region 326 to increase PD of the low PDoutlier template 320L, referred to as a modified template 320LA. A size(and pitch), shape and type of sub-resolution dummy features 430 arechosen to make the PD of the modified template 320LA satisfies the PDtarget 310 with the PD target range.

Referring again to FIG. 1, the method 100 proceeds to step 108C byperforming an exposure process to the semiconductor substrate 120 usingthe modified template 320LA.

Additional steps can be provided before, during, and after the method100, and some of the steps described can be repeated, replaced,eliminated, or moved around for additional embodiments of the method100. For example, steps from 104 to 108A, 108B and 106C may be repeatedto bring PD of the template 124 to satisfy a new PD target with a new PDtarget range.

Based on the above, the present disclosure offers a method forfabricating a semiconductor device. The method employs identifying a PDoutlier template, splitting the PD outlier template into a subsettemplate and performing a PD-outlier-treatment on the subset template toeliminate PD outlier template and improve PD uniformity. The method alsoemploys performing individual exposure process using respective thesubset of template. The method demonstrates reducing space charge effectin e-beam lithography, improving uniformity of pattern density,improving process window and throughput.

Thus, the present disclosure provides one embodiment of an integratedcircuit (IC) method. The IC method includes receiving pattern densities(PDs) with a first PD range r of a plurality of templates of an ICdesign layout, identifying a high PD outlier template and a low PDoutlier template from the plurality of templates, splitting the high PDoutlier template into multiple subsets of template, wherein each subsetof template carries a portion of PD of the high PD outlier template,performing a PD uniformity (PDU) optimization to the low PD outliertemplate and performing multiple individual exposure processes usingrespective subset of templates.

The present disclosure provides another embodiment of an IC method. Themethod includes receiving pattern densities (PDs) with a first range rof a plurality of templates of an IC design layout, determining a PDtarget and a second range R, wherein the second range R is smaller thanthe first range r, wherein the PD target is chosen to be smaller than amaximum PD defined by an e-beam blur budget, according to the PD targetand the second range R, identifying high PD outlier templates and low PDoutlier templates from the plurality of the template, splitting the highPD outlier template into multiple subsets of templates. Each subset oftemplates carries a portion of PD of the high PD outlier template and asum of PDs of each subset is equal to 100% of the PD of the PD outliertemplate. The method also includes inserting sub-resolution dummyfeature in the low PD outlier templates and performing multipleindividual exposure processes using the respective subset of templates.

The present disclosure also provides another embodiment of an IC method.The method includes receiving pattern densities (PDs) with a first ranger of a plurality of templates of an IC design layout, determining a PDtarget and a second range R. The second range R is smaller than thefirst range R. The method also includes, according to the PD target andthe second range R, identifying a high PD outlier template, which has aPD higher than the PD target, and a low PD outlier template, which has aPD lower than the PD target. The method also includes splitting the highPD outlier region into subsets of template, wherein each subset oftemplate has a PD satisfies the PD target, or is within—the second rangeR The method also includes performing PD uniformity (PDU) optimizationto the second subset of templates and performing a first exposureprocess to the first subsets of template and a second exposure processto the second subset of templates, which uses a different exposure dosethan the first exposure process.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method of semiconductor device fabrication,comprising: receiving an integrated circuit (IC) layout patternincluding a plurality of templates; identifying, from the plurality oftemplates, a first template having a first layout pattern with a firstpattern density (PD) and a second template having a second layoutpattern with a second PD less than the first PD; splitting the firsttemplate into a plurality of subset templates, wherein each subsettemplate of the plurality of subset templates includes a portion of thefirst layout pattern, and wherein each subset template has a subset PDthat satisfies a PD target; performing a PD uniformity (PDU)optimization to the second template; and performing multiple individualelectron beam (e-beam) lithography exposure processes with an e-beamlithography tool to a semiconductor substrate, using respective ones ofthe subset templates, thereby patterning the semiconductor substrate. 2.The method of claim 1, further comprising: receiving the IC layoutpattern, wherein each template of the plurality of templates includes aportion of the IC layout pattern; prior to identifying the firsttemplate and the second template, determining a PD for each template ofthe plurality of templates, wherein the determining the PD provides afirst range of pattern densities.
 3. The method of claim 2, wherein thefirst template and the second template are identified by: determiningthe PD target having a second range of pattern densities, wherein thesecond range of pattern densities is less than the first range ofpattern densities; and based on the PD target and the second range ofpattern densities, defining the first template and the second template,wherein the first template has the first PD which is greater than the PDtarget and also out of the second range of pattern densities, andwherein the second template has the second PD which is less than the PDtarget and also out of the second range of pattern densities.
 4. Themethod of claim 1, wherein the PD target is chosen to be less than a PDdefined by an e-beam blur budget.
 5. The method of claim 3, wherein thefirst template includes a first region and a second region, wherein thefirst region has a third PD and a first critical dimension (CD), andwherein the second region has a fourth PD less than the third PD and asecond CD less than the first CD.
 6. The method of claim 5, wherein thefirst region is split into a first subset template and the second regionis split into a second subset template.
 7. The method of claim 6,wherein the PDU optimization is performed by inserting a sub-resolutiondummy feature into the portion of the IC layout pattern defined by thesecond subset template.
 8. The method of claim 6, wherein performing themultiple individual e-beam lithography exposure processes includes:performing a first e-beam lithography exposure process to thesemiconductor substrate using the first subset template, wherein thefirst e-beam lithography exposure process is performed using a firstexposure dose; and after performing the PDU optimization, performing asecond e-beam lithography exposure process to the semiconductorsubstrate using the second subset template, wherein the second e-beamlithography exposure process is performed using a second exposure dosewhich is different than the first exposure dose.
 9. The method of claim3, wherein the first template includes a third region and a fourthregion, wherein the third region has a third PD and a first CD, andwherein the fourth region has a fourth PD less than the third PD and asecond CD less than the first CD.
 10. The method of claim 9, wherein thethird region is split into a third subset template and a fourth subsettemplate such that: the third subset template includes X₁% of the thirdPD of the third region; the fourth subset template includes X₂% of thethird PD of the third region and 100% of the fourth PD of the fourthregion; a total of the X₁% of the third PD plus the X₂% of the third PDis equal to 100% of the third PD of the third region; and the X₁% of thethird PD of the third region and the X₂% of the third PD of the thirdregion each satisfy the PD target, or are within the second range ofpattern densities.
 11. The method of claim 10, wherein performing themultiple individual e-beam lithography exposure processes includes:performing a third e-beam lithography exposure process to thesemiconductor substrate using the third subset template, wherein thethird e-beam lithography exposure process is performed using a firstexposure dose; and performing a fourth e-beam lithography exposureprocess to the semiconductor substrate using the fourth subset template,wherein the fourth e-beam lithography exposure process is performedusing a second exposure dose which is different than the first exposuredose.
 12. The method of claim 9, wherein the third region is split intoa fifth subset template and a sixth subset template and the fourthregion is split into a seventh subset template such that: the fifthsubset template includes X₃% of the third PD of the third region; thesixth subset template includes X₄% of the third PD of the third region;the seventh subset template includes 100% of the fourth PD of the fourthregion; a total of the X₃% of the third PD plus the X₄% of the third PDis equal to 100% of the third PD of the third region; and the X₃% of thethird PD of the third region and the X₄% of the third PD of the thirdregion each satisfy the PD target, or are within the second range of thepattern densities.
 13. A method of semiconductor device fabrication,comprising: receiving an integrated circuit (IC) layout patternincluding a plurality of templates; determining a pattern density (PD)target having a second range of pattern densities, wherein the PD targetis chosen to be less than a PD defined by an e-beam blur budget; basedon the PD target and the second range of pattern densities, identifying,from the plurality of templates, a first template having a first layoutpattern with a first PD and a second template having a second layoutpattern with a second PD less than the first PD; splitting the firsttemplate into a plurality of subset templates, wherein each subsettemplate of the plurality of subset templates includes a portion of thefirst layout pattern and a percentage of the first PD, wherein a totalof the percentages of the first PD of each subset template adds up to100% of the first PD of the first template; and wherein each subsettemplate has a subset PD that satisfies the PD target; inserting asub-resolution dummy feature into the portion of the IC layout patterndefined by the second template to perform a PD uniformity (PDU)optimization to the second template; and performing multiple individualelectron beam (e-beam) lithography exposure processes with an e-beamlithography tool to a semiconductor substrate, using respective ones ofthe subset templates, thereby patterning the semiconductor substrate.14. The method of claim 13, further comprising: receiving the IC layoutpattern, wherein each template of the plurality of templates includes aportion of the IC layout pattern; prior to determining the PD target,determining the PD for each template of the plurality of templates,wherein the determining the PD provides a first range of patterndensities; and determining the PD target, wherein the second range ofpattern densities is less than the first range of pattern densities. 15.The method of claim 13, wherein the first template includes a firstregion and a second region, wherein the first region has a third PD anda first critical dimension (CD), and wherein the second region has afourth PD less than the third PD and a second CD less than the first CD.16. The method of claim 15, wherein the first region is split into afirst subset template and a second subset template such that: the firstsubset template includes X₁% of the third PD of the first region; thesecond subset template includes X₂% of the third PD of the first regionand 100% of the fourth PD of the second region; a total of the X₁% ofthe third PD plus the X₂% of the third PD is equal to 100% of the thirdPD of the first region; and the X₁% of the third PD of the first regionand the X₂% of the third PD of the first region each satisfy the PDtarget, or are within the second range of pattern densities.
 17. Themethod of claim 16, wherein performing the multiple individual e-beamlithography exposure processes includes: performing a first e-beamlithography exposure process to the semiconductor substrate using thefirst subset template, wherein the first e-beam lithography exposureprocess is performed using a first exposure dose; and performing asecond e-beam lithography exposure process to the semiconductorsubstrate using the second subset template, wherein the second e-beamlithography exposure process is performed using a second exposure dosewhich is different than the first exposure dose.
 18. The method of claim15, wherein the first region is split into a third subset template and afourth subset template and the second region is split into a fifthsubset template such that: the third subset template includes X₃% of thethird PD of the first region; the fourth subset template includes X₄% ofthe third PD of the first region and the fifth subset template includes₁₀₀% of the fourth PD of the second region: a total of the X₃% of thethird PD plus the X₄% of the third PD is equal to 100% of the third PDof the first region; and the X₃% of the third PD of the first region andthe X₄% of the third PD of the first region each satisfy the PD target,or are within the second range of pattern densities.
 19. A method ofsemiconductor device fabrication, comprising: receiving an integratedcircuit (IC) layout pattern including a plurality of templates;determining a pattern density (PD) target having a second range ofpattern densities; based on the PD target and the second range ofpattern densities, identifying, from the plurality of templates, a firsttemplate including a first layout pattern with a first PD and a secondtemplate including a second layout pattern with a second PD less thanthe first PD; splitting the first template into a plurality of subsettemplates including at least a first subset template and a second subsettemplate, wherein each subset template of the plurality of subsettemplates has a subset PD that satisfies the PD target; performing a PDuniformity (PDU) optimization to the second template; and performing afirst e-beam lithography exposure process with an e-beam lithographytool to the semiconductor substrate using the first subset template andperforming a second e-beam lithography exposure process to thesemiconductor substrate using the second subset templates, wherein eachof the first and second e-beam lithography exposure processes usedifferent exposure doses.
 20. The method of claim 19, furthercomprising: receiving the IC layout pattern, wherein each template ofthe plurality of templates includes a portion of the IC layout pattern;prior to determining the PD target, determining a PD for each templateof the plurality of templates, wherein the determining the PD provides afirst range of pattern densities; determining the PD target, wherein thesecond range of pattern densities is less than the first range ofpattern densities; identifying the first template, wherein the first PDis greater than the PD target, and the second template, wherein thesecond PD is less than the PD target; and splitting the first template,wherein each subset template of the plurality of subset templates has asubset PD that is within the second range of pattern densities.